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Our celestial neighbor holds an unending number of mysteries. What happened to Mars' atmosphere? What did an ancient Mars look like? Could the planet have supported life? Now there's a new question puzzling the scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif.,—Where did that rock come from?

The photo above, taken from the Opportunity rover, compares the same terrain on the Martian surface only 12 sols (Martian days) apart. In what is possibly the easiest spot-the-difference puzzle game ever, scientists noticed that a rock the size of a "jelly doughnut" had somehow appeared in front of the rover.

"It was a total surprise," said lead scientist Steve Squyres in an interview with Discovery News. "We were like, 'wait a second, that wasn't there before, it can't be right. Oh my god! It wasn't there before!' We were absolutely startled." The scientists then gave the rock a name, "Pinnacle Island."

Assuming they're not the victims of a Martian prank, JPL scientists have two plausible ideas on the rock's unknown origin. The rock could have been flipped over during one of Opportunity's maneuvers. The second (and less likely) conclusion is that a nearby meteorite sent Pinnacle Island spiraling into the rover's path.

However, the rock's origin story isn't the only thing that has the JPL team excited. "[The rock] obligingly turned upside down," said Squyres. "We're seeing a side that hasn't seen the Martian atmosphere in billions of years, and there it is for us to investigate. It's just a stroke of luck."

Opportunity continues to be an example in fortitude. Originally scheduled for a 3-month mission, the little rover is still trucking a decade later and has clocked nearly 23 miles in its journey. This rock could represent Opportunity's last Martian mystery, but for now, it's still searching.